When I fell pg by surprise, I was a 15 a day smoker (8mg), and had been smoking for 4 years. It took me until about week 12-14 of my pg to give up completely.
First I cut down to 4mg, and then 2mg, and would also ration myself out less and less each day.
It wasn't easy, especially as DP was still smoking around me all throughout my pg, but I somehow I managed to stay off them. (Although on many occasions I would drop to my knees and beg DP for a ciggy, and he would refuse to give me one... which was a good thing!)
I haven't gone back to them at all since Aidyn was born (tried one and though it was foul!) so I have now been smoke free for 2 years!
And I managed to set a good enough example for DP as well who has now been off them for a year.